unit-i introduction to i.c. engines
DESCRIPTION
IntroTRANSCRIPT
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1
Unit-I
Introduction to Internal Combustion Engines
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2
Historical and Modern Development
Nomenclature
Classification and Comparison of SI and CI engines
4 stroke & 2 stroke engines
First Law analysis, Energy Balance.
Outline of the Unit-I
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2
Introduction
Heat engine:
It can be defined as any engine that converts thermal energy
to mechanical work output.
Examples of heat engines include: steam engine, diesel
engine, and gasoline (petrol) engine.
On the basis of how thermal energy is being delivered to
working fluid of the heat engine, heat engine can be
classified as an internal combustion engine and external
combustion engine.
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In an Internal combustion engine, combustion takes place
within working fluid of the engine, thus fluid gets contaminated with
combustion products.
Petrol engine is an example of internal combustion engine,
where the working fluid is a mixture of air and fuel .
In an External combustion engine, working fluid gets energy
using boilers by burning fossil fuels or any other fuel, thus the
working fluid does not come in contact with combustion products.
Steam engine is an example of external combustion
engine, where the working fluid is steam.
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Introduction Contd
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Internal combustion engines may be classified as :
Spark Ignition engines.
Compression Ignition engines.
Spark ignition engine (SI engine): An engine in which the
combustion process in each cycle is started by use of an external
spark.
Compression ignition engine (CI engine): An engine in which
the combustion process starts when the air-fuel mixture self
ignites due to high temperature in the combustion chamber
caused by high compression.
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Introduction Contd
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Spark ignition and Compression Ignition engine operate
on either a four stroke cycle or a two stroke cycle.
Four stroke cycle: It has four piston strokes over two
revolutions for each cycle.
Two stroke cycle: It has two piston strokes over one
revolution for each cycle.
We will be dealing with Spark Ignition engine and
Compression Ignition engine operating on a four stroke
as well as two stroke cycle.
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Introduction Contd
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Fig.1 : Engine components [W. W. Pulkrabek] 7
Introduction Contd
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Introduction Contd
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Fig 2: Engine Parts 9
Introduction Contd
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Internal combustion Engine Components
I.C. Engine components shown in fig.1 and fig. 2 are defined as
follows:
1. Block: Body of the engine containing cylinders, made of
cast iron or Aluminium.
2. Cylinder: The circular cylinders in the engine block in which
the pistons reciprocate back and forth.
3. Head: The piece which closes the end of the cylinders,
usually containing part of the clearance volume of the
combustion chamber.
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Internal combustion Engine Components Contd
4. Combustion chamber: The end of the cylinder between the
head and the piston face where combustion occurs.
The size of combustion chamber continuously changes
from minimum volume when the piston is at TDC to a
maximum volume when the piston at BDC.
5. Crankshaft: Rotating shaft through which engine work output
is supplied to external systems.
The crankshaft is connected to the engine block with
the main bearings.
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It is rotated by the reciprocating pistons through the
connecting rods connected to the crankshaft, offset from the
axis of rotation. This offset is sometimes called crank throw
or crank radius.
6. Connecting rod: Rod connecting the piston with the rotating
crankshaft, usually made of steel or alloy forging in most
engines but may be aluminum in some small engines.
7. Piston rings: Metal rings that fit into circumferential grooves
around the piston and form a sliding surface against the cylinder
walls.
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Internal combustion Engine Components Contd
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8. Camshaft: Rotating shaft used to push open valves at the
proper time in the engine cycle, either directly or through
mechanical or hydraulic linkage (push rods, rocker arms,
tappets) .
9. Push rods: The mechanical linkage between the camshaft and
valves on overhead valve engines with the camshaft in the
crankcase.
10.Crankcase: Part of the engine block surrounding the
crankshaft. In many engines the oil pan makes up part of the
crankcase housing.
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Internal combustion Engine Components Contd
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11. Exhaust manifold: Piping system which carries exhaust gases
away from the engine cylinders, usually made of cast iron.
12. Intake manifold: Piping system which delivers incoming air to
the cylinders, usually made of cast metal, plastic, or composite
material.
In most SI engines, fuel is added to the air in the intake
manifold system either by fuel injectors or with a
carburetor.
The individual pipe to a single cylinder is called runner.
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Internal combustion Engine Components Contd
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13.Carburetor: A device which meters the proper amount of
fuel into the air flow by means of pressure differential.
For many decades it was the basic fuel metering system
on all automobile (and other) engines.
14.Spark plug: Electrical device used to initiate combustion
in an SI engine by creating high voltage discharge across
an electrode gap.
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Internal combustion Engine Components Contd
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15.Exhaust System: Flow system for removing exhaust gases
from the cylinders, treating them, and exhausting them to the
surroundings.
It consists of an exhaust manifold which carries the exhaust
gases away from the engine, a thermal or catalytic converter to
reduce emissions, a muffler to reduce engine noise, and a
tailpipe to carry the exhaust gases away from the passenger
compartment.
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Internal combustion Engine Components Contd
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16.Flywheel: Rotating mass with a large moment of inertia
connected to the crank shaft of the engine.
The purpose of the flywheel is to store energy and furnish
large angular momentum that keeps the engine rotating
between power strokes and smooths out engine operation.
17.Fuel injector: A pressurized nozzle that sprays fuel into the
incoming air (SI engines )or into the cylinder (CI engines).
18.Fuel pump: Electrically or mechanically driven pump to supply
fuel from the fuel tank (reservoir) to the engine.
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Internal combustion Engine Components Contd
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19.Glow plug: Small electrical resistance heater mounted
inside the combustion chamber of many CI engines, used
to preheat the chamber enough so that combustion will
occur when first starting a cold engine.
The glow plug is turn off after the engine is started.
20.Starter: Several methods are used to start IC engines.
Most are started by use of an electric motor (starter)
geared to the engine flywheel. Energy is supplied from an
electric battery.
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Internal combustion Engine Components Contd
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Fig. 3: Engine Terminology 19
Engine Terminology
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Engine Terminology
Fig. 4: Engine Terminology
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The engine terminology are explained as follows:
1. Top Dead Center (TDC):
Position of the piston when it stops at the furthest point
away from the crankshaft.
Top because this position is at the top of the engines
(not always), and dead because the piston stops as this
point. Because in some engines TDC is not at the top of
the engines(e.g.: horizontally opposed engines, radial
engines etc.).
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Engine Terminology
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Some sources call this position Head End Dead Center
(HEDC).
Some source call this point TOP Center (TC).
When the piston is at TDC, the volume in the cylinder is a
minimum called the clearance volume.
2. Bottom Dead Center (BDC):
Position of the piston when it stops at the point closest to the
crankshaft.
Some sources call this Crank End Dead Center (CEDC)
because it is not always at the bottom of the engine. Some
source call this point Bottom Center (BC).
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Engine Terminology
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3. Stroke: Distance traveled by the piston from one extreme position
to the other : TDC to BDC or BDC to TDC.
4. Bore: It is defined as cylinder diameter or piston face diameter;
piston face diameter is same as cylinder diameter( minus small
clearance).
5. Swept volume/Displacement volume
Volume displaced by the piston as it travels through one
stroke.
Swept volume is defined as stroke times bore.
Displacement can be given for one cylinder or entire engine
(one cylinder times number of cylinders).
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Engine Terminology
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6. Clearance volume
It is the minimum volume of the cylinder available for the
charge (air or air fuel mixture) when the piston reaches at
its outermost point (top dead center or outer dead center)
during compression stroke of the cycle.
Minimum volume of combustion chamber with piston at
TDC.
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Engine Terminology
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7. Compression ratio
The ratio of total volume to clearance volume of the cylinder
is the compression ratio of the engine.
Typically compression ratio for SI engines varies form 8 to
12 and for CI engines it varies from 12 to 24.
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Engine Terminology
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SI Engine & Ideal Otto Cycle
We will be dealing with four stroke SI engine.
The following figure shows the PV diagram of
Ideal Otto cycle.
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Fig. 4: Suction stroke 30
Working Principle of S.I. Engine
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1. Suction/Intake Stroke:
Intake of air fuel mixture in cylinder through intake manifold.
The piston travel from TDC to BDC with the intake valve
open and exhaust valve closed.
This creates an increasing volume in the combustion
chamber, which in turns creates a vacuum.
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Working Principle of S.I. Engine Contd
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The resulting pressure differential through the intake system
from atmospheric pressure on the outside to the vacuum on
the inside causes air to be pushed into the cylinder.
As the air passes through the intake system fuel is added to it
in the desired amount by means of fuel injectors or a
carburetor.
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Working Principle of S.I. Engine Contd
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Fig.5: Compression Stroke 33
Working Principle of S.I. Engine Contd
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2. Compression stroke:
When the piston reaches BDC, the intake valve closes and the
piston travels back to TDC with all valves closed.
This compresses air fuel mixture, raising both the pressure
and temperature in the cylinder.
Near the end of the compression stroke the spark plug is
fired and the combustion is initiated.
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Working Principle of S.I. Engine Contd
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Combustion of the air-fuel mixture occurs in a very short but finite
length of time with the piston near TDC (i.e., nearly constant volume
combustion).
It starts near the end of the compression stroke slightly before
TDC and lasts into the power stroke slightly after TDC.
Combustion changes the composition of the gas mixture to
that of exhaust products and increases the temperature in the
cylinder to a high value.
This in turn increases the pressure in the cylinder to a high
value.
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Working Principle of S.I. Engine Contd
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Fig. 6: Combustion followed by Expansion stroke. 36
Working Principle of S.I. Engine Contd
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3. Expansion stroke/Power stroke
With all valves closed the high pressure created by the combustion
process pushes the piston away from the TDC.
This is the stroke which produces work output of the engine
cycle.
As the piston travels from TDC to BDC, cylinder volume is
increased, causing pressure and temperature to drop.
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Working Principle of S.I. Engine Contd
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Fig.7: Exhaust blowdown followed by Exhaust stroke 38
Working Principle of S.I. Engine Contd
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Exhaust Blowdown:
Late in the power stroke, the exhaust valve is opened and exhaust
blowdown occurs.
Pressure and temperature in the cylinder are still high relative
to the surroundings at this point, and a pressure differential is
created through the exhaust system which is open to
atmospheric pressure.
This pressure differential causes much of the hot exhaust gas
to be pushed out of the cylinder and through the exhaust
system when the piston is near BDC.
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Working Principle of S.I. Engine Contd
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This exhaust gas carries away a high amount of enthalpy,
which lowers the cycle thermal efficiency.
Opening the exhaust valve before BDC reduces the work
obtained but is required because of the finite time needed for
exhaust blowdown.
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Working Principle of S.I. Engine Contd
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4. Exhaust stroke
By the time piston reaches BDC, exhaust blowdown is complete, but
the cylinder is still full of exhaust gases at approximately
atmospheric pressure.
With the exhaust valve remaining open, the piston travels from
BDC to TDC in the exhaust stroke.
This pushes most of the remaining exhaust gases out of the
cylinder into the exhaust system at about atmospheric
pressure, leaving only that trapped in the clearance volume
when the piston reaches TDC.
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Working Principle of S.I. Engine Contd
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Near the end of the exhaust stroke before TDC, the intake
valve starts to open, so that it is fully open by TDC when the
new intake stroke starts the next cycle.
Near TDC the exhaust valve starts to close and finally is fully
closed sometime after TDC.
This period when both the intake valve and exhaust valve
are open is called valve overlap, it can be clearly seen in
valve timing chart given below.
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Working Principle of S.I. Engine Contd
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43
Working Principle of S.I. Engine Contd
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Compression Ignition Engine
We will deal with Compression Ignition engine.
The ideal diesel cycle PV diagram is shown in following
fig. 8.
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Fig.8: P-V Diagram 45
Ideal diesel cycle
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Fig.9: Four strokes of Ideal Diesel Cycle. 46
Working Principle of C.I. Engine
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Fig.10: Suction stroke 47
Working Principle of C.I. Engine Contd
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Fig.11: Compression stroke 48
Working Principle of C.I. Engine Contd
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1. Intake/Suction Stroke
The same as the intake stroke in an SI engine with one major
difference : no fuel is added to the incoming air, refer fig. 10.
2. Compression Stroke
The same as in an SI engine except that only air is compressed and
compression is to higher pressures and temperature, refer fig.11.
Late in the compression stroke fuel is injected directly into the
combustion chamber, where it mixes with very hot air.
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Working Principle of C.I. Engine Contd
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This causes the fuel to evaporate and self ignite, causing
combustion to start.
Combustion is fully developed by TDC and continues at about
constant pressure until fuel injection is complete and the piston
starts towards BDC, refer fig.12.
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Working Principle of C.I. Engine Contd
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Fig.12:Fuel injection and combustion followed by Expansion stroke 51
Working Principle of C.I. Engine Contd
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Fig.13: Exhaust blowdown followed by exhaust stroke 52
Working Principle of C.I. Engine Contd
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3. Expansion/Power stroke
The power stroke continues as combustion ends and the
piston travels towards BDC, refer fig. 12.
Exhaust blowdown same as with an SI engine.
4. Exhaust stroke
Same as with an SI engine, refer fig. 13.
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Working Principle of C.I. Engine Contd
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Working Principle of C.I. Engine Contd
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Comparison of SI and CI Engines
Description SI Engine CI Engine
Basic Cycle It works on Otto cycle or constant
volume heat addition cycle.
It works on Diesel cycle or constant
pressure heat cycle.
Fuel Gasoline, a highly volatile fuel. Self-
ignition temperature is high.
Diesel oil, a non-volatile fuel. Self-
ignition temperature is comparatively
low.
Introduction of fuel A mixture of air and fuel is
introduced during the piston's
suction stroke. A carburetor and an
ignition system are necessary.
Modern engines have gasoline
injection.
Fuel is injected directly into the
combustion chamber at high pressure at
the end of the compression stroke. A fuel
pump and injector are necessary.
Load control Throttle controls the quantity of fuel-
air mixture to control the load.
The quantity of fuel is regulated to
control the load. Air quantity is not
controlled.
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Description SI Engine CI Engine
Ignition Requires an ignition system with spark
plug in the cylinder head of the engine.
The voltage is provided to the spark plug
either from the battery or from the
magneto.
When air is compressed to high pressures,
its temperature also increases beyond the
self-ignition temperature of the fuel. Hence,
the ignition of fuel occurs due to
compression of the air and there is no need
for spark plugs.
Compression ratio 6 to 10. Upper limit is fixed by antiknock
quality of the fuel.
16 to 20. Upper limit is fixed by weight
increase of the engine.
Thermal efficiency The lower compression ratio reduces their
potential to achieve higher thermal
efficiency.
The value of compression ratio is higher;
hence these engines have the potential to
achieve higher thermal efficiency.
Speed SI engines are lightweight, and the fuel is
homogeneously burned, hence achieving
very high speeds.
CI engines are heavier and the fuel is
burned heterogeneously, hence producing
lower speeds.
Weight Light weight Heavier in comparison to SI engines.
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Comparison of SI and CI Engines
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Two Stroke
Internal Combustion Engines
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Two Stroke Engines
Everything a 4 stroke
engine does in 2
revolutions a 2 stroke
engine does in 1
revolution of the
crankshaft.
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Applications of Two Stroke Engines
This type of engine is commonly found in applications such
as:
1. Lawn and garden equipment
2. Old motorbikes
3. Diesel engines in ships etc.
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Introduction to the Two Stroke Engine
Two stroke engines have:
Simplified construction (no valves).
Fire once every revolution for a significant power
boost.
Great power to weight ratio.
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Two Stroke Engines part names
Piston
Cylinder
Crankshaft Connecting
Rod
Still uses a flywheel
(not shown) Combustion
chamber
Intake port
Exhaust port
Reed valve
Transfer port
Crankcase
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Working Principle of Two Stroke Engine
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The two stroke engine ignites every revolution of the
crankshaft.
360 degrees rotation of crankshaft completes the cycle.
In a two stroke engine we have only:
Compression
Combustion Thus, Two Strokes.
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Working Principle of Two Stroke Engine Contd
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Working Principle of Two Stroke Engine Contd
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TDC BDC
Piston moves from
BDC to TDC
Air/Fuel/Oil mixture is sucked into crankcase
Reed Valve
Is sucked
open
Working Principle of Two Stroke Engine Contd
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1. Intake stroke
The fuel/air mixture is first
drawn into the crankcase by
the vacuum created during the
upward stroke of the piston
through the reed valve.
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Working Principle of Two Stroke Engine Contd
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Two Stroke
Piston BDC to TDC.
A/F/O mixture sucked into crankcase.
Four Stroke
Piston TDC to BDC.
A/F mixture sucked into cylinder.
Working Principle of Two Stroke Engine Contd
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TDC BDC
Piston gets to
TDC
Air/Fuel/Oil mixture is now trapped in crankcase.
Reed Valve
Shuts
Working Principle of Two Stroke Engine Contd
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TDC BDC
Piston moves back
To BDC
Air/Fuel/Oil mixture is now pressurized in crankcase called primary compression.
Reed Valve
sealing
Working Principle of Two Stroke Engine Contd
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Crankcase Compression
Is only a few pounds of pressure per square inch (psi) [very
weak].
Cylinder compression in a four stroke engine was several psi
[very strong].
The crankcase in a two stroke engine has to be very small so
we can build some pressure when the piston is moving to
BDC.
Working Principle of Two Stroke Engine Contd
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Crankcase
Compression
What is going to happen when the top of piston gets to here?
Working Principle of Two Stroke Engine Contd
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A/F/O mixture squirts into cylinder because of crankcase compression
Working Principle of Two Stroke Engine Contd
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Piston reaches BDC and Cylinder fills with A/F/O mixture.
Working Principle of Two Stroke Engine Contd
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Another A/F/O mixture is sucked into crankcase while First one is compressed in cylinder.
Working Principle of Two Stroke Engine Contd
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2. Compression stroke
The piston then rises, driven by
flywheel momentum, and
compresses the fuel
mixture. (At the same time,
another intake stroke is
happening beneath the piston).
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Working Principle of Two Stroke Engine Contd
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TDC BDC
Piston gets to
TDC
Air/Fuel/Oil mixture is ignited in cylinder.
Piston is pounded
Down the cylinder
Working Principle of Two Stroke Engine Contd
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3. Power stroke
At the top of the stroke the
spark plug ignites the fuel-air
mixture.
The burning fuel expands,
driving the piston downward.
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Working Principle of Two Stroke Engine Contd
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4. Exhaust/Transfer stroke
Toward the end of the stroke, the
piston exposes the intake port,
allowing the compressed fuel/air
mixture in the crankcase to escape
around the piston into the main
cylinder.
This expels the exhaust gasses out
the exhaust port, usually located on
the opposite side of the cylinder.
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Working Principle of Two Stroke Engine Contd
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Working Principle of Two Stroke Engine Contd
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1. Two-stroke engines do not have valves, which simplifies their
construction and lowers their weight.
2. Hence less maintenance problems.
3. Two-stroke engines fire once every revolution, while four-
stroke engines fire once every other revolution. This gives
two-stroke engines a significant power boost which is twice
theoretically as compared to four stroke engines.
4. The work required to overcome the friction of the exhaust and
suction strokes is saved.
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Advantage of Two Stroke Engines
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Advantage of Two Stroke Engines
5. More uniform turning moment is obtained on the crankshaft
and hence a lighter flywheel is required.
6. For the same output, two strokes engines occupy lesser
space.
7. In case of two-stroke engines, burnt gases do not remain in
clearance space as in case of four strokes engines.
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Disadvantages of a Two Stroke Engines
1. The engines do not last as long as four stroke does due to
poor lubrication.
2. You have to mix two cycle engine oil with gasoline.
3. The engines do not use fuel efficiently.
4. These engines produce a lot of pollution.
5. With heavy loads, two stroke engines get heated due to
excessive heat produced.
6. Consumption of lubrication is more in two stroke engines.
7. Effective compression is less in two stroke engines. 82
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Comparison of Four Stroke and Two Stroke Engines
Four Stroke Engine Two Stroke Engines
One power stroke for every two
revolutions of the crankshaft.
One power stroke for each revolution of
the crankshaft.
There are inlet and exhaust valves in
the engine.
There are inlet and exhaust ports instead
of valves.
Crankcase is not fully closed and air
tight.
Crankcase is fully closed and air tight.
Top of the piston compresses the
charge.
Both sides of the piston compress the
Charge.
Size of the flywheel is comparatively
larger.
Size of the flywheel is comparatively
smaller.
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Four Stroke Engine Two Stroke Engines
Thermal efficiency is higher. Part
load efficiency is better.
Thermal efficiency is comparatively
low. Part load efficiency is poor.
For a given weight, engine would
give only half the power of two
stroke.
For same weight, two stroke engine
gives twice the power that of four
stroke engine.
Two stroke engines are great for the power to weight ratio and
their simple design, however, due to there pollution concerns
these engines will be harder to find.
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Comparison of Four Stroke and Two Stroke Engines
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85 Fig. 15. Energy flow through reciprocating engine
The First Law Analysis of Engine Cycle
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86 Fig. 16. Reciprocating engine as an open system
The First Law Analysis of Engine Cycle
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87
The fuel is fed into the combustion chamber where it burns in
air converting chemical energy of fuel into heat.
Some part of heat is lost through the engine exhaust, to the
coolant and due to radiation.
The heat energy which is converted to power on the top of the
piston is called the indicated power ip.
A part of energy is lost due to bearing friction, pumping losses
etc. and a part of energy available is utilized in driving the
auxiliary devices like feed pump, valve mechanisms, ignition
system etc.
The First Law Analysis of Engine Cycle
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The First Law Analysis of Engine Cycle
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The sum of all these losses, expressed in units of power
is termed as frictional power fp.
The remaining energy is the useful mechanical energy
and is termed as the brake power, bp.
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Internal Combustion engine fundamentals by John B. Heywood
Engineering Fundamentals of the Internal combustion Engine by
Willard W. Pulkrabek
Internal combustion engines by V Ganesan
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mages/72180.jpg
http://www.howcarswork.co.uk/modules/articles/index.php?cat_id
=1
http://www.howcarswork.co.uk/modules/content/index.php?id=23
References
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http://www.small-engines.com/4cycleth.html
http://en.wikipedia.org/wiki/Engine_displacement
http://en.wikipedia.org/wiki/Stroke_(engine)
http://en.wikipedia.org/wiki/Internal_combustion_engine
http://www.howstuffworks.com/diesel-two-stroke.htm
http://www.mustangmonthly.com/techarticles/97278_how_engines
_work/index.html
http://www.kruse-ltc.com/Otto/otto_cycle.php
http://www.kruse-ltc.com/Diesel/diesel_cycle.php
References
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http://www.answers.com/topic/internal-combustion-engine
http://www.britannica.com/EBchecked/topic/162716/diesel-engine
http://content.answers.com/main/content/img/BritannicaConcise/i
mages/72180.jpg
http://www.howcarswork.co.uk/modules/articles/index.php?cat_id
=1
References
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Thank You
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